Award-winning American author Ray Bradbury, best known for such work as "Fahrenheit 451" and "The Martian Chronicles", has died at the age of 91, his family said on Wednesday.

President Barack Obama led praise for Bradbury, seen as one of the genre's greatest authors, who died Tuesday in Los Angeles after an unspecified "lengthy illness" according to his publisher HarperCollins.

"But Ray also understood that our imaginations could be used as a tool for better understanding, a vehicle for change, and an expression of our most cherished values."

Director Steven Spielberg added: "He was my muse for the better part of my sci-fi career. He lives on through his legion of fans. In the world of science fiction and fantasy and imagination he is immortal.

Bradbury's most famous work, 1953's "Fahrenheit 451," was a Cold War-era warning of the evils of censorship and thought control in a totalitarian state. It reached a worldwide audience as a film adapted by Francois Truffaut in 1966.

"In a career spanning more than seventy years, Ray Bradbury has inspired generations of readers to dream, think, and create," HarperCollins said in a statement.

He was not the first to examine the dual potential for good and bad in science and technology, but he sought out a larger audience.

Before Bradbury, science fiction had mostly been published in pulp magazines, aiming for mass-circulation magazines such as Mademoiselle and The Saturday Evening Post.

He helped bring modern science fiction into the literary mainstream. More than eight million copies of his books have been sold in 36 languages.

Ray Douglas Bradbury was born August 22, 1920 -- an event he claimed to remember -- in Waukegan, Illinois, the third son of a telephone lineman and Swedish immigrant Esther Marie Bradbury.

The family moved to Los Angeles, where Bradbury attended Los Angeles High School and joined the drama club with plans to become an actor.

He graduated in 1938, but skipped university in favor of independent study at a local library, reading Leo Tolstoy and Fyodor Dostoyevsky and others, while selling newspapers on the street.

His first paycheck as a writer came for a short story, "Pendulum," published in Super Science Stories, a pulp magazine.

He published his first book, "Dark Carnival," in 1947, the year he married Marguerite McClure.

Bradbury preferred the label fantasy to "sci-fi," defining it as "a depiction of the unreal" and giving as an example "The Martian Chronicles," because it was a story that could not happen.

"Fahrenheit 451" was his only sci-fi book, he said, because it was a "depiction of the real" -- or of something that could actually happen in a totalitarian state.

Bradbury said the novel -- named after the temperature at which printed books ignite -- was not meant to be grim: "I wasn't trying to predict the future. I was trying to prevent it."

"You don't have to burn books to destroy a culture. Just get people to stop reading them," he once said.

Bradbury branched out into film, television and theater, with an Academy Award nomination for his 1962 animated film, "Icarus Montgolfier Wright," and an Emmy as a television writer for "The Halloween Tree."

In his private life he was terrified of flying, and, surprisingly for someone who wrote about the future, Bradbury rejected computers -- he insisted on using a typewriter, saying PCs were too quiet -- and the Internet.

"People are talking about the Internet as a creative tool for writers. I say, "B.S. Stay away from that. Stop talking to people around the world and get your work done," he told Playboy magazine in 1996, aged 75.

Flowers were placed Tuesday on his sidewalk star on the Hollywood Walk of Fame in Los Angeles. He was also immortalized by having an asteroid named after him: 9766 Bradbury.

"The sound I hear today is the thunder of a giant's footsteps fading away. But the novels and stories remain, in all their resonance and strange beauty," said writer Stephen King, cited by the Hollywood Reporter.

Bradbury is survived by his four daughters and eight grandchildren. His wife, Marguerite, died in 2003, after fifty-seven years of marriage.